.\" Copyright 2016, IBM Corporation.
.\" Written by Mike Rapoport <rppt@linux.vnet.ibm.com>
.\" Copyright 2016, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Copyright 2024, Alejandro Colomar <alx@kernel.org>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH UFFDIO_API 2const 2024-06-17 "Linux man-pages 6.9.1"
.SH NAME
UFFDIO_API
\-
enable operation of the userfaultfd and perform API handshake
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.BR "#include <linux/userfaultfd.h>" "  /* Definition of " UFFD* " constants */"
.B #include <sys/ioctl.h>
.P
.BI "int ioctl(int " fd ", UFFDIO_API, struct uffdio_api *" argp );
.P
.B #include <linux/userfaultfd.h>
.P
.fi
.EX
.B struct uffdio_api {
.BR "    __u64 api;" "       /* Requested API version (input) */"
.BR "    __u64 features;" "  /* Requested features (input/output) */"
.BR "    __u64 ioctls;" "    /* Available ioctl() operations (output) */"
.B };
.EE
.SH DESCRIPTION
Enable operation of the userfaultfd and perform API handshake.
.P
The
.I api
field denotes the API version requested by the application.
The kernel verifies that it can support the requested API version,
and sets the
.I features
and
.I ioctls
fields to bit masks representing all the available features and the generic
.BR ioctl (2)
operations available.
.P
Since Linux 4.11,
applications should use the
.I features
field to perform a two-step handshake.
First,
.B UFFDIO_API
is called with the
.I features
field set to zero.
The kernel responds by setting all supported feature bits.
.P
Applications which do not require any specific features
can begin using the userfaultfd immediately.
Applications which do need specific features
should call
.B UFFDIO_API
again with a subset of the reported feature bits set
to enable those features.
.P
Before Linux 4.11, the
.I features
field must be initialized to zero before the call to
.BR UFFDIO_API ,
and zero (i.e., no feature bits) is placed in the
.I features
field by the kernel upon return from
.BR ioctl (2).
.P
If the application sets unsupported feature bits,
the kernel will zero out the returned
.I uffdio_api
structure and return
.BR EINVAL .
.P
The following feature bits may be set:
.TP
.BR UFFD_FEATURE_EVENT_FORK " (since Linux 4.11)"
When this feature is enabled,
the userfaultfd objects associated with a parent process are duplicated
into the child process during
.BR fork (2)
and a
.B UFFD_EVENT_FORK
event is delivered to the userfaultfd monitor
.TP
.BR UFFD_FEATURE_EVENT_REMAP " (since Linux 4.11)"
If this feature is enabled,
when the faulting process invokes
.BR mremap (2),
the userfaultfd monitor will receive an event of type
.BR UFFD_EVENT_REMAP .
.TP
.BR UFFD_FEATURE_EVENT_REMOVE " (since Linux 4.11)"
If this feature is enabled,
when the faulting process calls
.BR madvise (2)
with the
.B MADV_DONTNEED
or
.B MADV_REMOVE
advice value to free a virtual memory area
the userfaultfd monitor will receive an event of type
.BR UFFD_EVENT_REMOVE .
.TP
.BR UFFD_FEATURE_EVENT_UNMAP " (since Linux 4.11)"
If this feature is enabled,
when the faulting process unmaps virtual memory either explicitly with
.BR munmap (2),
or implicitly during either
.BR mmap (2)
or
.BR mremap (2),
the userfaultfd monitor will receive an event of type
.BR UFFD_EVENT_UNMAP .
.TP
.BR UFFD_FEATURE_MISSING_HUGETLBFS " (since Linux 4.11)"
If this feature bit is set,
the kernel supports registering userfaultfd ranges on hugetlbfs
virtual memory areas
.TP
.BR UFFD_FEATURE_MISSING_SHMEM " (since Linux 4.11)"
If this feature bit is set,
the kernel supports registering userfaultfd ranges on shared memory areas.
This includes all kernel shared memory APIs:
System V shared memory,
.BR tmpfs (5),
shared mappings of
.IR /dev/zero ,
.BR mmap (2)
with the
.B MAP_SHARED
flag set,
.BR memfd_create (2),
and so on.
.TP
.BR UFFD_FEATURE_SIGBUS " (since Linux 4.14)"
.\" commit 2d6d6f5a09a96cc1fec7ed992b825e05f64cb50e
If this feature bit is set, no page-fault events
.RB ( UFFD_EVENT_PAGEFAULT )
will be delivered.
Instead, a
.B SIGBUS
signal will be sent to the faulting process.
Applications using this
feature will not require the use of a userfaultfd monitor for processing
memory accesses to the regions registered with userfaultfd.
.TP
.BR UFFD_FEATURE_THREAD_ID " (since Linux 4.14)"
If this feature bit is set,
.I uffd_msg.pagefault.feat.ptid
will be set to the faulted thread ID for each page-fault message.
.TP
.BR UFFD_FEATURE_PAGEFAULT_FLAG_WP " (since Linux 5.10)"
If this feature bit is set,
userfaultfd supports write-protect faults
for anonymous memory.
(Note that shmem / hugetlbfs support
is indicated by a separate feature.)
.TP
.BR UFFD_FEATURE_MINOR_HUGETLBFS " (since Linux 5.13)"
If this feature bit is set,
the kernel supports registering userfaultfd ranges
in minor mode on hugetlbfs-backed memory areas.
.TP
.BR UFFD_FEATURE_MINOR_SHMEM " (since Linux 5.14)"
If this feature bit is set,
the kernel supports registering userfaultfd ranges
in minor mode on shmem-backed memory areas.
.TP
.BR UFFD_FEATURE_EXACT_ADDRESS " (since Linux 5.18)"
If this feature bit is set,
.I uffd_msg.pagefault.address
will be set to the exact page-fault address that was reported by the hardware,
and will not mask the offset within the page.
Note that old Linux versions might indicate the exact address as well,
even though the feature bit is not set.
.TP
.BR UFFD_FEATURE_WP_HUGETLBFS_SHMEM " (since Linux 5.19)"
If this feature bit is set,
userfaultfd supports write-protect faults
for hugetlbfs and shmem / tmpfs memory.
.TP
.BR UFFD_FEATURE_WP_UNPOPULATED " (since Linux 6.4)"
If this feature bit is set,
the kernel will handle anonymous memory the same way as file memory,
by allowing the user to write-protect unpopulated page table entries.
.TP
.BR UFFD_FEATURE_POISON " (since Linux 6.6)"
If this feature bit is set,
the kernel supports resolving faults with the
.B UFFDIO_POISON
ioctl.
.TP
.BR UFFD_FEATURE_WP_ASYNC " (since Linux 6.7)"
If this feature bit is set,
the write protection faults would be asynchronously resolved
by the kernel.
.P
The returned
.I argp->ioctls
field can contain the following bits:
.\" FIXME This user-space API seems not fully polished.  Why are there
.\" not constants defined for each of the bit-mask values listed below?
.TP
.B 1 << _UFFDIO_API
The
.B UFFDIO_API
operation is supported.
.TP
.B 1 << _UFFDIO_REGISTER
The
.B UFFDIO_REGISTER
operation is supported.
.TP
.B 1 << _UFFDIO_UNREGISTER
The
.B UFFDIO_UNREGISTER
operation is supported.
.SH RETURN VALUE
On success,
0 is returned.
.P
On error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EFAULT
.I argp
refers to an address that is outside the calling process's
accessible address space.
.TP
.B EINVAL
The API version requested in the
.I api
field is not supported by this kernel, or the
.I features
field passed to the kernel includes feature bits that are not supported
by the current kernel version.
.TP
.B EINVAL
A previous
.B UFFDIO_API
call already enabled one or more features for this userfaultfd.
Calling
.B UFFDIO_API
twice,
the first time with no features set,
is explicitly allowed
as per the two-step feature detection handshake.
.TP
.B EPERM
The
.B UFFD_FEATURE_EVENT_FORK
feature was enabled,
but the calling process doesn't have the
.B CAP_SYS_PTRACE
capability.
.SH STANDARDS
Linux.
.SH HISTORY
Linux 4.3.
.SH CAVEATS
If an error occurs,
the kernel may zero the provided
.I uffdio_api
structure.
The caller should treat its contents as unspecified,
and reinitialize it before re-attempting another
.B UFFDIO_API
call.
.SH BUGS
In order to detect available userfault features and
enable some subset of those features
the userfaultfd file descriptor must be closed after the first
.B UFFDIO_API
operation that queries features availability and reopened before
the second
.B UFFDIO_API
operation that actually enables the desired features.
.SH EXAMPLES
See
.BR userfaultfd (2).
.SH SEE ALSO
.BR ioctl (2),
.BR ioctl_userfaultfd (2),
.BR mmap (2),
.BR userfaultfd (2)
.P
.I linux.git/\:Documentation/\:admin\-guide/\:mm/\:userfaultfd.rst
